1. Field of the Invention
The present invention relates to a fluorinated polyimide optical material and more particularly to a fluorinated polyimide optical material having a low transmission loss in near-infrared region which can be used as an optical material for light guiding in opto-electronic integrated circuits or implemented optical-electrical mixed wiring boards. Furthermore, the present invention relates to a fluorinated polyimide which can be used as a major component of such an optical material, to a fluorinated poly(amic acid) which is an intermediate or precursor for a fluorinated polyimide, and to fluorinated tetracarboxylic acids or their dianhydride, which are starting compounds for the preparation of the fluorinated poly(amic acid), as well as to methods for preparing the fluorinated polyimide, fluorinated poly(amic acid), and starting compounds, respectively.
2. Description of the Related Arts
Plastic materials are generally lighter in weight than inorganic materials and are featured by a higher impact strength, a higher processability, easier handling and the like and they have heretofore been used widely for various optical purposes such as optical fibers or lenses, substrates for optical discs. When plastics are to be used as media for transmitting near-infrared lights for optical transmission such as optical waveguides for opto-electronic integrated circuits (OEIC), optical electronic mixed implemented wiring boards, a problem arises that plastics have high optical losses as compared with inorganic materials. Causes of light transmission loss in plastics are roughly classified into two factors, i.e., scattering and absorption. According as the wavelengths of lights used in light transmission are shifted toward longer wavelength regions (from 0.85 .mu.m to 1,0 .mu.m.about.1.7 .mu.m) the latter cause, i.e., optical loss ascribable to high harmonic absorption of infrared vibration which is inherent to the molecular structure of the material will become dominant, and thus it is feared that use of plastics in light transmission applications would be difficult. In particular, poly(methyl methacrylate) (PMMA) and polystyrene (PS) which have been widely used as an optical material for a visible light region have two or more types of carbon-to-hydrogen bonds (C--H bonds) in the molecular chain and hence there are a plurality of broad, strong absorption peaks in near-infrared absorption spectra. To decrease the intensity of harmonics absorption due to C--H bonds by shifting it toward longer wavelength region, it has been indicated that substitution of hydrogens in the molecule with deuterium (D) or fluorine (F) is effective, and there have already been made fundamental studies on PMMA and PS materials of which hydrogens were substituted with deuterium or fluorine (cf., e.g., Toshikuni Kaino, Appl. Phys. Lett., 48, No.12, p.757 (1986)). However, these plastic optical materials have insufficient soldering resistances (260.degree. C.) required for the fabrication of OEIC on a silicone substrate, which necessitates various devices about fabrication steps when they are applied to OEIC and the like.
On the other hand, polyimides generally have a thermal decomposition initiation temperature of 400.degree. C. or higher and are known as one of those having the highest thermal resistance among the plastics, and their application to optical materials has recently come to be studied. (cf. e.g., H. Franke, J. D. Crow, SPIE, Vol. 651, Integrated Optical Circuit Engineering III, pp. 102-107 (1986); and C. T. Sullivan, SPIE, Vol. 994, p.92 (1988)).
Further, a fluorine-containing polyimide coating material comprising a polyimide having a hexafluoroisopropylidene group has been studied for its feasibility as a thermal resistant coating material having an improved clarity (Anne K. St. Clair and Wayne S. Slemp, SAMPE Journal, July/August, pp. 28-33 (1985)). On the other hand, with view to decreasing optical losses, there have been proposed optical waveguides comprising a fluorine-containing polyimide having a hexafluoroisopropylidene group in its main chain (cf. Rainer Reuter, Hilmar Franke, and Claudius Feger, Applied Optics, Vol. 27, No. 21, pp.4565-4571 (1988)).
However, as far as is known, all the polyimides including fluorine-containing polyimides thus far proposed or available have C--H bonds in phenyl groups in the polymer chain and therefore their absorption spectra in near-infrared region contain peaks which can be assigned to harmonics due to the stretching vibration of C--H bonds or to a combination of harmonics due to the stretching vibration and deformation vibration of C--H bonds. As a result, low optical loss over the entire range of optical transmission wavelength region (1.0 to 1.7 .mu.m) has remained to be achieved.
Accordingly, theoretically perdeuteration or perfluorination of polyimide will reduce optical losses in optical transmission wavelength region. However, as far as is known there has been no report on the synthesis of perdeuterated or perfluorinated polyimides. Perdeuteration would seem insufficient for decreasing absorption peaks over the entire optical transmission wavelength region because third harmonics due to C--D bond appear near a wavelength of 1.5 .mu.m.
In summary, no plastics optical material has been known that fulfils both requirements of a high optical transmission over the entire optical wavelength region and a high thermal resistance simultaneously.